Hi,
Just tried it with NEF, NRW, CR2 and works great. Just a couple of suggestions: enumerate all installed codecs, gather the supported file extensions from each codec and build your filter dynamically, so when a new codec gets written your plugin just sees it automagically (and you also support all existing codecs right away). I did this with my Photoshop WIC Import Plugin (which does the same thing as yours, but for Photoshop). You can filter out on the Vendor GUID if you want to exclude stock codecs from your scan: Microsoft's VendorGUID is {F0E749CA-EDEF-4589-A73A-EE0E626A2A2B}, but then you might inadvertently exclude the PSD codec that ships with Blend 3...
Besides that the codec pack's URL is http://www.fastpictureviewer.com/codecs/
You can find sample raw files at rawsamples.ch, raw.fotosite.pl and imaging-resource.com
Thank you for emphasizing the donationware status of the codecs. Cannot wait to see them in action with Paint.NET, thanks to your plugin!
Axel (codec's author)